In the UDL window (Language > Define Your Language) there’s a field near the top marked "Ext. ". Type in the file extension you want to be associated with your UDL. You can use a common one like txt, or make up your own - but don’t include the dot.
In your case, just enter adoc.
As for how to make N++ start in a UDL - I don’t think there is a way to do that.